1. Field of the Invention
The invention relates to a mobile station positioning method and a positioning system and a mobile station implementing the method. In addition, the invention relates to a method for transmitting mobile station position information from a mobile station to a base station and to a mobile communication system, and a mobile station implementing the method. The method can be advantageously applied in a digital time-division mobile communication system, such as the GSM (Global System for Mobile Communications) mobile telephone network.
2. Description of the Prior Art
The position information of a mobile station can be used for many purposes:
pricing of calls can be performed according to the position of a mobile station, whereby calls made from a home environment, for example, can be cheaper;
when an emergency call is made from a mobile station, it is possible to determine the position of that mobile station;
the user of a mobile station may need information about his/her position e.g. when travelling,
police and other authorities can use the position information to locate a stolen mobile station or to trace a missing person, for example.
In addition, a mobile communication system can utilize the position information when changing serving base stations. Up to the present, the change of serving base stations has required the use of other information measured from the transmission channel, such as signal attenuation on the transmission channel, because the position information available is not accurate enough for the purpose. Apart from the accuracy, the position information used for the change of serving base stations has to be nearly real-time, so the position measurement has to be performed quickly and at short intervals.
There are several known methods to locate a mobile station. According to the prior art, the mobile communication system knows which is the base station in whose coverage area the mobile station is located, and so the position of the mobile station can be determined with the accuracy of roughly the cell size in the system. Since the cell size in the system may be large, say 100 km.sup.2, the accuracy of this position information is too modest for most applications.
A method is known from publication [1] WO 92/05672 wherein the distance of a mobile station from a base station is determined on the basis of the propagation delay of a signal transmitted between the mobile station and the base station. FIG. 1 illustrates the operating principle of such a method. By measuring the propagation delay of a signal between a base station 1 and a mobile station 14, it is obtained a distance estimate d.sub.1 for the distance between the mobile station and the base station. Because of a certain measuring accuracy of the distance measurement, the assumed location of the mobile station is a ring-shaped area the width of which depends on the time delay measuring accuracy. A corresponding propagation delay measurement can be performed for a signal between the mobile station and other base stations. The result is one ring-shaped location area per each base station used in the measurement. In the situation illustrated by FIG. 1, the propagation delays between the mobile station and base station 11, mobile station and base station 12, and mobile station and base station 13 produce distance estimates d.sub.1, d.sub.2 and d.sub.3 which correspond to the ring-shaped areas 16, 17 and 18. The rings intersect at area 19, which is the mobile station location area obtained from the three propagation delay measurements. Thus, the location of the mobile station can be determined to be an area whose order of dimension is the measuring accuracy.
The method based on a propagation delay measured from a signal between a mobile station and a base station has the disadvantage that the mobile station has to establish a connection with every base station for which the propagation delay is to be measured. Then the operating range for the position measurement stays small because in most cases it is not possible to establish connections to a sufficient number of base stations because of long distances, attenuating obstacles or low transmission power, for example. If the position information has to be continuously updated, the transmission of the position information puts a considerable load on the connection capacity of the mobile communication system. In addition, the high amount of data transmitted makes the measuring slow. It is also a problem of the method that errors in the absolute timing accuracy of the mobile station produce errors in the positioning result obtained.
In addition, a method is known from publication [2] EP 398773 wherein a mobile station receives from a mobile communication system information about clocks and location coordinates of base stations situated in the vicinity of the mobile station. Then the mobile station measures the time differences of the signals received from said base stations and determines the position of the mobile station on the basis of measured time differences, base station clocks and position coordinates and the position location algorithm stored in the mobile station. The publication does not describe in detail how the method would be implemented in a mobile communication system. The principle of measurement based on time differences is described in more detail in connection with the description of the method according to the invention.
The method described in publication [2] has the disadvantage that the mobile station has to perform a considerable amount of processing related to the calculation of position coordinates, which takes away processor capacity from other functions of the mobile station. Furthermore, the algorithms of the location method used in the mobile communication system have to be stored in the mobile station, whereby it becomes very difficult to make changes in the method since the mobile station base of the system needs then to be updated. In addition, there is the disadvantage that it is primarily the system, and not the mobile station, that needs the position information, which means that the position coordinates calculated by the mobile station have to be transmitted from the mobile station to the system. Similarly, the base station must continuously transmit information to the mobile station about the position coordinates and clocks of the nearest base stations. As the position information contains a fair amount of data, the transmission of said information would considerably consume the transmission capacity of the system.
Problem to be Solved
According to what has been expressed above, mobile station position information can be applied in many ways, but the characteristics of the prior art solutions do not meet the requirements of the applications.
Object of the Invention
The object of the invention is to provide a method and a system for determining the position information of a mobile station, and a mobile station implementing the method, avoiding the aforementioned disadvantages of the prior art.